The present invention relates to manufacture of frozen aerated products, particularly ice creams, and also to use of extrusion procedures.
A conventional process for manufacturing aerated frozen products, in particular ice cream, comprises the operations of blending, homogenizing, pasteurizing, freezing and hardening the mixture to be frozen. Aeration of the mixture or expansion is carried out during the freezing step in a proportion such that the volume increases by 70 to 120%. On leaving the freezer, the temperature of the aerated mass is typically -5 to -6.degree. C. The mass is then hardened at -40 to -45.degree. C. in a hardening chamber until the temperature of the product reaches -18.degree. C. or lower at the core for bulk products or -30.degree. C. for products extruded as bars.
An attempt has been made to reduce the temperature of the mass leaving the freezer for reasons of energy saving and with the object of improving the texture, for example in the direction of improved oiliness. Insurmountable problems have however been encountered with conventional equipment with the high viscosity of the ice cream mass at temperatures below -7 to -8.degree. C. These problems have been partially resolved by using two freezers with a scraped surface in series, the first being a conventional one delivering aerated ice cream at approximately -7.degree. C., and the second being specially designed to treat the highly viscous material to lower its temperature to approximately -10.degree. C.
Within the same type of ideas, European Patent Application Publication No. 0 561 118 describes a three-step process for producing ice cream at a low temperature, down to -20.degree. C., which is an exit temperature so that a hardening step may be entirely dispensed with for bulk products and considerably be shortened for extruded products. Further, in a first step, a so-called pre-expansion step, air is incorporated into the mixture to be frozen at a positive temperature. In a second step, the aerated mass is cooled in a scraped surface exchanger and leaves at approximately -6.degree. C. During the third step, a screw device cools the mass to approximately -20.degree. C.
U.S. Pat. No. 5,024,066 also discloses a two-step system. Pre-expansion takes place in the first step during which air is added to the mass to be frozen at a positive temperature. In the second step the aerated mass is cooled by means of an archimedean screw with a rough surface provided with scraping knives at its periphery, to a negative temperature sufficiently low to ensure a stable texture of the frozen mass, which enables the products to be stored directly in a cold chamber.